A feast called the Conception of Mary arose in the Eastern Church in the seventh century. It came to the West in the eighth century. In the eleventh century it received its present name, the Immaculate Conception. In the eighteenth century it became a feast of the universal Church.
In 1854 Pius IX gave the infallible statement: "The most Blessed Virgin Mary, in the first instant of her conception, by a singular grace and privilege granted by almighty God, in view of the merits of Jesus Christ, the savior of the human race, was preserved free from all stain of original sin."
In Luke 1:28 the angel Gabriel, speaking on God's behalf, addresses Mary as "full of grace" (or "highly favored"). In that context this phrase means that Mary is receiving all the special divine help necessary for the task ahead. However, the Church grows in understanding with the help of the Holy Spirit. The Spirit led the Church, especially non-theologians, to the insight that Mary had to be the most perfect work of God next to the Incarnation. Or rather, Mary's intimate association with the Incarnation called for the special involvement of God in Mary's whole life. The logic of piety helped God's people to believe that Mary was full of grace and free of sin from the first moment of her existence. Moreover, this great privilege of Mary is the highlight of all that God has done in Jesus. Rightly understood, the incomparable holiness of Mary shows forth the incomparable goodness of God.